Encrypt megolm keys securely in indexeddb

[ara4n]

Currently we encrypt megolm keys with a hardcoded key of DEFAULT_KEY, as per https://github.com/matrix-org/matrix-js-sdk/blob/master/src/crypto/OlmDevice.js#L77

Instead we should do something like https://crypto.stackexchange.com/a/52488 to generate a secure keypair, and store it in Indexed DB with the .extractable property set to false.

[bwindels]

wrt the linked ticket, as the CryptoKey would also be stored in the network home directory (within indexeddb), wouldn't that still allow the admin to run Riot off that directory and decrypt the messages?

[uhoreg]

Closing in favour of #7271